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High pressure fluid phase equilibria

Du Rand, Marlie (2000-12)

Thesis (MScEng)--University of Stellenbosch, 2000.

Thesis

ENGLISH ABSTRACT: Supercritical extraction is being investigated as a possible alternative to the processes
currently used in the fractionation of paraffinic waxes. By removing the lighter carbon
fractions from the wax, the wax hardness will be improved and its melting temperature
range reduced, hence improving the performance of the wax product in certain
applications. In order to evaluate and operate such an extraction process optimally, it is
necessary to have a thermodynamic model that accurately represents the process system.
There are, however, currently no predictive models available for these systems. In order to
fit present models to the systems, accurate phase equilibrium data of the supercritical
solvent - n-alkane systems are needed. Unfortunately, the amount of reliable published
data on these systems in the required operating range is very limited.
A view cell was designed and developed with which these high pressure equilibria could
be studied. The binary phase equilibria of supercritical CO2 with n-CI2, n-CI6, n-C20, n-C24,
n-C28 and n-C36 and of supercritical ethane with n-CI6, n-C24 and n-C28 were measured in
the temperature range 313 - 367 K. It was found that the systems with these two solvents
have very different types of phase behaviour. The n-alkane solubility is much higher in
ethane, but supercritical CO2 will provide a much better degree of control over the
selectivity achieved in an extraction process.
Of the various equations of state investigated, it was found that the Patel Teja equation of
state provided the best fit of the CO2 - n-alkane systems and that the Soave-Redlich-
Kwong equation fitted the ethane - n-alkane systems the best. The interaction parameters
of both these equations of state display a functional relationship with temperature and nalkane
acentric factor, making it possible to determine parameter values for application at
other operating temperatures and with other n-alkane systems.
It was found that the current equations of state were not able to represent the phase
equilibria accurately over the entire range of operating conditions. The poor performance
of the equations of state can be attributed to inherent flaws in the existing equations of
state.